Method of surface treatment for stainless steel product for fuel cell

ABSTRACT

A method for treating the surface of a stainless steel product for a fuel cell containing, in wt %, 0.15% or less of C, 17 to 36% of Cr, 0.005 to 3.5% of B, which comprises the first step of forming in advance a passive film with an oxidizing acid on the surface of the stainless steel product, the second step of allowing an aqueous acid solution to corrode the passive film, to thereby project one or more of a M 23 C 6  type carbide, a M 23 (C, B) 6  type borocarbide and M 2 B type boride, which are inclusions having good electroconductivity, the third step of forming a passive coating film with an oxidizing acid on the surface of the steel product except that of the inclusion above projected, and the fourth step of washing with water and drying. The method of treatment allows, without the use of a method requiring a high cost such as gold plating, the improvement in the contact resistance of the stainless steel product for a fuel cell, which results in the retention of excellent characteristics of a stainless steel product over a long period of time.

TECHNICAL FIELD

[0001] The present invention relates to a method of surface treatmentfor a stainless steel product for fuel cells, and more particularly,relates to a technique of reducing the contact resistance by protrudingconductive inclusions on the surface and maintaining a low contactresistance for a long period by preventing fall off of conductiveinclusions.

BACKGROUND ART

[0002] For example, a separator of a fuel cell forms a reaction gaspassage, and functions as a conductor for supplying an electric currentgenerated by contacting with an electrode to the outside, and it ishence required to be low in contact resistance. Recently, from theviewpoint of strength and corrosion resistance, fuel cell separatorsmade of stainless steel are drawing attention. Stainless steel productsare superior in corrosion resistance because a passive film is formed onthe surface; however, the passive film is high in electrical resistance.Accordingly, in Japanese Patent Application Laid-open (JP-A) No.10-228914, it is proposed to reduce the electrical resistance by platingthe separator contact area with gold.

[0003] In this proposal, however, because of gold plating, the separatorbecomes very expensive, and the manufacturing cost of the fuel cells ishigher. In JP-A No. 11-121018, a separator having carbon particlesburied in the surface of a stainless steel product is proposed. However,this separator easily falls off due to friction caused by vibration orthe like, and is not suitable for long-term use.

DISCLOSURE OF THE INVENTION

[0004] It is hence an object of the invention to provide a method ofsurface treatment for stainless steel products for fuel cells which iscapable of enhancing the contact resistance without resort to costlytechniques such as plating, and exhibiting the superior features ofstainless steel product for a long period.

[0005] The present inventors first researched surface treatment methodsforrotruding conductive inclusions contained in stainless steel on thesurface. In this surface treatment method, limited quantities of C and Bare contained in a stainless steel product to precipitate highlyconductive inclusions such as carbide, boron carbide, and boride, andthe surface of the stainless steel product is etched by an acidicaqueous solution to protrude the conductive inclusions on the surface,and the surface is treated with an oxidizing acid to form a passivefilm. According to this surface treatment method, since the conductiveinclusions are protruding from the passive film, the contact resistanceis lowered as the conductive inclusions act as contact points, whilecorrosion resistance is not sacrificed.

[0006] In such a surface treatment method, however, conductiveinclusions easily fall off from the matrix, and the contact resistanceincreases with the passing of the time. According to the research by theinventors, reasons for such inconvenience are estimated to be asfollows.

[0007]FIG. 1 is electron microscope photographs of a stainless steelproduct treated by this surface treatment method. The photographs showthat the matrix surrounding the conductive inclusions is excavated. Thisis illustrated in FIG. 3, and it is found that gaps are formed aroundthe conductive inclusions due to crevice corrosion or pitting corrosion.That is, in the surface treatment method for etching the surface with anacidic aqueous solution first, when the conductive inclusionsprecipitate, Cr is taken into the inner side, and the surrounding Crconcentration is diluted, and, as a result, at the time of etching withthe acidic aqueous solution, Fe components and Ni components around theconductive inclusions elute selectively, and slight gaps are formedbetween the conductive inclusions and the matrix. Consequently, apassive film is formed on the surface of the matrix by an oxidizingacid, but crevice corrosion or pitting corrosion is initiated from thealready formed slight gaps. Therefore, in this surface treatment method,the adhering strength of conductive inclusions to the matrix is weak,and they may easily fall off from the matrix due to friction during useor at the time of bending processing or the like. Even if the conductiveinclusions do not fall off, the electrical resistance is increased,since the contact area between the conductive inclusions and matrix issmall.

[0008] The method of surface treatment for stainless steel products forfuel cells of the invention is an improved method of such surfacetreatment methods, and is applied in a manufacturing method of stainlesssteel products for fuel cells, comprising, by weight, C: 0.15% or less,Cr: 17 to 36%, and B: 0.005 to 3.5%, which comprises a first step offorming a passive film preliminarily on the surface of the stainlesssteel product by an oxidizing acid, a second step of protruding one ormore types of conductive inclusions M₂₃C₆ type carbide, M₂₃(C, B)₆ typeboron carbide, and M₂B type boride on the surface by etching the passivefilm in an acidic aqueous solution, a third step of forming a passivefilm on the surface other than the inclusions protruding on the etchedsurface by an oxidizing acid, and a fourth step of washing in water anddrying.

[0009] Generally, on the surface of a stainless steel product, a passivefilm mainly composed of Cr hydroxide is formed by contact with the airin its manufacturing process, but the passive film formed on thestainless steel product surface in the air is very thin and is aboutseveral nanometers. On the other hand, the Cr concentration in thematrix around the conductive inclusions is lowered along withprecipitation of inclusions, and hence a sound film as a passive film isnot formed sufficiently. The inventors completed the invention bydiscovering that a sound passive film can be formed by repairing theregion not forming a sound film as a passive film around the conductiveinclusions into a sound passive film by adding an oxidizing acid at thefirst step, thereby forming a Cr-rich passive film. In other words, byrestoring the passive film around the conductive inclusions to form asound film in the first step, when etched in an acidic aqueous solutionin the second step, the surrounding of the conductive inclusions is notselectively eluted and excavated, and gaps as the cause of crevicecorrosion are hardly formed in the matrix, and the invention iscompleted by the determination of this fact.

[0010] Moreover, in the second step of the invention, since the Fecomponent and Ni component in the passive film elute preferentially, theCr concentration in the passive film is increased, and the passive filmbecomes rich in Cr of higher corrosive resistance. In the third step, apassive film is further formed by an oxidizing acid, and the Cr oxideconcentration becomes higher, so that the corrosion resistance may befurther enhanced.

[0011]FIG. 2 is electron microscope photographs of a stainless steelproduct treated by the surface treatment method of the invention, andFIG. 4 is a schematic diagram showing a cross section of conductiveinclusions. As is clear from these diagrams, it is known that crevicecorrosion or pitting corrosion does not take place between theconductive inclusions and the matrix. Thus, according to the surfacetreatment method of the invention, occurrence of crevice corrosion orthe like around the conductive inclusions can be prevented. Therefore,the contact area between the conductive inclusions and the matrix can beincreased, and the electrical resistance can be lowered. It is alsoeffective to prevent fall off of conductive inclusions, and a lowelectrical resistance can be maintained for a long period. Also in thesecond step, a Cr-rich passive film can be formed, and the corrosionresistance can be enhanced.

[0012] Moreover, a de-B layer is hardly formed by using a brightannealed steel material, as compared with air annealing, since it isannealed in a non-oxidizing atmosphere. As a result, it is effective toarrest decrease of conductive inclusions exposed after pickling.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1A and FIG. 1B are electron microscope photographs of thesurface of a stainless steel product for fuel cells treated by a surfacetreatment method of etching the surface in an acidic aqueous solutionfirst.

[0014]FIG. 2A and FIG. 2B are electron microscope photographs of thesurface of a stainless steel product for fuel cells treated by thesurface treatment method of the invention.

[0015]FIG. 3 is a sectional view schematically showing the conductiveinclusions shown in FIG. 1.

[0016]FIG. 4 is a sectional view schematically showing the conductiveinclusions shown in FIG. 2.

[0017]FIG. 5 is a diagram showing contact resistance and passive stateholding current density at 0.9 V in an example of the invention.

[0018]FIG. 6 is a diagram showing the relationship of accelerationduration time and contact resistance in an example of the invention.

[0019]FIG. 7 is a diagram showing the relationship of power generationtime and contact resistance in an example of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0020] Preferred embodiments of the invention are described below.

[0021] Austenitic stainless steel product

[0022] In the invention, an austenitic stainless steel product can beused. That is, a manufacturing method of a stainless steel product forfuel cells in a preferred embodiment of the invention is a surfacetreatment method of austenitic stainless steel products for fuel cellscomprising, by weight, C: 0.15% or less, Si: 0.01 to 1.5%, Mn: 0.01 to2.5%, P: 0.035% or less, S: 0.01% or less, Al: 0.001 to 0.2%, N: 0.3% orless, Cu: 0 to 3%, Ni: 7 to 50%, Cr: 17 to 30%, Mo: 0 to 7%, B: 0.005 to3.5%, and balance: Fe and inevitable impurities, with the contents ofCr, Mo, and B satisfying the following formula, which comprises a firststep of forming a passive film preliminarily on the surface of theaustenitic stainless steel product of the specified composition by anoxidizing acid, a second step of protruding one or more types ofconductive inclusions M₂₃C₆ type carbide, M₂₃(C, B)₆ type boron carbide,and M₂B type boride on the surface by etching the passive film in anacidic aqueous solution, a third step of forming a passive film on thesurface other than the inclusions protruding on the etched surface by anoxidizing acid, and a fourth step of washing in water and drying:

Cr(%)+3×Mo(%)−2.5×B(%)≧17

[0023] wherein element symbols denote contents (wt. %).

[0024] Ferritic stainless steel product

[0025] In the invention, a ferritic stainless steel product can be used.That is, a manufacturing method of a stainless steel product for fuelcells in the other preferred embodiment of the invention is a method ofsurface treatment for ferritic stainless steel products for fuel cellscomprising, by weight, C: 0.15% or less, Si: 0.01 to 1.5%, Mn: 0.01 to1.5%, P: 0.035% or less, S: 0.01% or less, Al: 0.001 to 0.2%, N: 0.35%or less, Cu: 0 to 1%, Ni: 0 to 5%, Cr: 17 to 36%, Mo: 0 to 7%, B: 0.005to 3.5%, and balance: Fe and inevitable impurities, with the contents ofCr, Mo, and B satisfying the following formula, which comprises a firststep of forming a passive film preliminarily on the surface of theferritic stainless steel product of the specified composition by anoxidizing acid, a second step of protruding one or more types ofconductive inclusions M₂₃C₆ type carbide, M₂₃(C, B)₆ type boron carbide,and M₂B type boride on the surface by etching the passive film in anacidic aqueous solution, a third step of forming a passive film on thesurface other than the inclusions protruding on the etched surface by anoxidizing acid, and a fourth step of washing in water and drying:

Cr(%)+3×Mo(%)−2.5×B(%)≧17

[0026] wherein element symbols denote contents (wt. %).

[0027] Conductive inclusions

[0028] Of the M₂₃C₆ type carbide, M₂₃(C, B)₆ type boron carbide metalinclusion, and M₂B type boride metal inclusion, the symbol “M” denotes ametal element, which is not a specific metal element, but is a metalelement with strong chemical affinity for C or B. Generally, in relationto coexisting elements in steel, M is mainly composed of Cr and Fe, andoften contains traces of Ni and Mo. In the case of carbide, B also hasan action as “M”. Metal inclusions of M₂₃C₆ type carbide include Cr₂₃C₆and (Cr, Fe)₂₃C₆, metal inclusions of M₂₃(C, B)₆ type boron carbideinclude Cr₂₃(C, B)₆ and (Cr, Fe)₂₃(C, B)₆, and metal inclusions of M₂Btype boride include Cr₂₃, (Cr, Fe)₂₃, (Cr, Fe, Ni)₂B, (Cr, Fe, Mo)₂B,(Cr, Fe, Ni, Mo)₂B, and Cr_(1.2)Fe_(0.76)Ni_(0.04)B. In any one of thesemetal inclusions of M₂₃C₆ type carbide or metal inclusions of M₂B typeboride, metal inclusions having part of C replaced by B oftenprecipitate, such as M₂₃(C, B)₈ type, M₄(C, B) type, M₂(C, B) type, M(C,B) type carbide metal inclusions and M₂(C, B) type boride metalinclusions, and these inclusions are supposed to be contained in theabove expression. Basically, dispersants of metals superior in electricconductivity are expected to have similar performances.

[0029] The reasons for limiting the numerical values of the contents ofthe components are as follows. In the following explanation, thepercentage refers to the percentage by weight.

[0030] C: 0.15% or less

[0031] C disperses and precipitates as a carbide mainly composed of Cr,and acts to lower the contact electrical resistance of the surface of astainless steel product covered with a passive film. However, if thecontent of C exceeds 0.15%, the strength and hardness are raised whilethe ductility is lowered, and the manufacturing efficiency reduces.Hence, the content of C is defined to be at 0.15% or less.

[0032] Cr: 17 to 36%

[0033] Cr is an important element for assuring the corrosion resistanceof the matrix, and its effect is insufficient if contained by less than17%. In contrast, if the Cr content exceeds 36%, manufacture on a massproduction scale is difficult. Hence, the content of Cr is defined to bewithin 17 to 36%. In the austenitic stainless steel, in particular, ifit exceeds 30%, the austenite phase becomes unstable, and hence it ispreferred to be 30% or less.

[0034] B: 0.005 to 3.5%

[0035] B precipitates as M₂₃(C, B)₆ type boron carbide and M₂B typeboride, and acts to lower the contact electrical resistance of thesurface of stainless steel product covered with a passive film. Suchaction is insufficient if the content of B is less than 0.005%. Incontrast, if the content of B exceeds 3.5%, on the other hand, it isdifficult to manufacture in an ordinary melting method, and the strengthand hardness become too high and the ductility drops, and hence theproductivity reduces. Hence, the content of B is defined within 0.005 to3.5%.

[0036] Cr(%)+3×Mo(%)−2.5×B(%)≧17

[0037] Due to precipitation of boride and carbide of Cr, the content ofCr in the steel contributing to corrosion resistance decreases ascompared with the content of Cr in the steel melting stage, and thecorrosion resistance of the matrix may deteriorate. In order to assurethe corrosion resistance inside the fuel cell, it is preferred that thecontent of Cr in the steel at least satisfy the relation in the formulaabove.

[0038] Si: 0.01 to 1.5%

[0039] Si, like Al, is an element for adding as an effective deoxidizer.If the content of Si is less than 0.01%, the deoxidizing action isinsufficient, and if it exceeds 1.5%, the forming performance islowered. Hence, it is preferred to contain Si in a range of 0.01 to1.5%.

[0040] Mn: 0.01 to 2.5%

[0041] Mn has an action of fixing S in the steel as a sulfide of Mn, andalso has an effect of improving the hot processing performance. In anaustenitic stainless steel product, Mn contributes to stabilization ofthe austenite phase. On the other hand, if the content of Mn exceeds2.5%, further effects are not expected. Hence, it is preferred tocontain Mn in a range of 0.01 to 2.5%. In the case of a ferriticstainless steel product, the content of Mn is sufficient at 1.5% orless.

[0042] P: 0.035% or less

[0043] P is an impurity element, and its content is preferred to be0.035% or less.

[0044] S: 0.01% or less

[0045] Almost S precipitates as Mn sulfide, Cr sulfide, Fe sulfide, orcomposite nonmetallic inclusion of their composite sulfides and oxides.Nonmetallic inclusions in any composition acts as starting points ofcorrosion, and is harmful for maintenance of passive film or suppressionof corrosion elution. Hence, the content of S should be defined to be at0.01% or less.

[0046] Al: 0.001 to 0.2%

[0047] Al is added in a steel melting stage as a deoxidizer. B is anelement having a strong bonding powder with oxygen in molten steel, andhence the oxygen concentration must be lowered by deoxidizing with Al.Hence, it is preferred to be contained in a range of 0.001 to 0.2%.

[0048] N: 0.3% or less, or 0.035% or less

[0049] In an austenitic stainless steel product, N is an effectiveelement for adjusting the balance of the austenite phase as an austeniteforming element. However, its upper limit should be 0.3% in order not todeteriorate the processing efficiency. In a ferritic stainless steelproduct, on the other hand, N is an impurity. N deteriorates toughnessat ordinary temperature, and hence the upper limit should be 0.035%.

[0050] Ni: 7 to 50%, or 0 to 5%

[0051] In an austenitic stainless steel product, Ni is an importantalloy element for stabilizing the austenite phase. If the content of Niis less than 7%, such action is insufficient, and if it exceeds 50%, itis difficult to manufacture. Hence, in the case of an austeniticstainless steel product, the content of Ni is preferred to be in a rangeof 7 to 50%.

[0052] In a ferritic stainless steel product, Ni is effective to improvethe corrosion resistance and toughness. If, however, the content of Niexceeds 5%, a two-phase texture of ferrite and austenite is formed, anddirectivity occurs in forming of thin plate, and sufficient processingefficiency is not obtained. Hence, in the case of a ferritic stainlesssteel product, the content of Ni is preferred to be in a range of 0 to5%.

[0053] Mo: 0 to 7%

[0054] Mo is effective to improve the corrosion resistance by a smallercontent than Cr. Preferably, Mo is contained at 7% or less as required.If it is contained at more than 7%, intermetallic compounds such assigma phase, making materials brittle, are more likely to precipitate.Hence, it is preferred to contain Mo in a range of 0 to 7%.

[0055] Cu: 0 to 3%, or 0 to 1%

[0056] Cu is an austenite phase stabilizing element, and functionseffectively for maintaining the passive state. If the content of Cuexceeds 3%, the hot processability is lowered, and mass production isdifficult. Hence, in an austenitic stainless steel product, it ispreferred to contain Cu in a range of 0 to 3%. In contrast, in aferritic stainless steel product, Cu exhibits a similar action, but theupper limit is preferably 1%.

[0057] Meanwhile, in order to form a favorable passive film, theoxidizing acid is preferred to contain at least nitric acid at 2 to 30%.To protrude conductive inclusions securely from the passive film byetching sufficiently, the acidic aqueous solution is preferred tocontain at least hydrofluoric acid at 2 to 20% and nitric acid at 5 to20%.

[0058] As the condition of bright annealing treatment for decreasing thede-B layer forming in the surface layer, the dew point temperature of−55 to −35° C. is preferred in a non-oxidizing atmosphere. Anon-oxidizing atmosphere is, for example, ammonia gas, anitrogen-hydrogen atmosphere, or a hydrogen atmosphere, and anitrogen-hydrogen atmosphere is particularly preferred. The annealingconditions are 1050 to 1120° C. for 20 seconds.

EXAMPLES

[0059] Referring now to exemplary embodiments, the invention is morespecifically described below.

[0060] A. Preparation of samples

[0061] An austenitic stainless steel product in the composition shown inTable 1 was melted in a vacuum melting furnace, formed into ingots, andhot rolled, cold rolled, and bright annealed for 20 seconds at 1080° C.in a nitrogen-hydrogen atmosphere, and a 0.15 mm thick sample wasfabricated. This sample was immersed for 10 minutes in an oxidizing acidof a 5% nitric acid solution held at 90° C., and a passive film wasformed. Consequently, the sample was immersed for 2 minutes in an acidicaqueous solution of a 10% nitric acid solution and a 4% hydrofluoricacid held at 60° C., and the sample surface was etched. This sample wasfurther immersed for 10 minutes in an oxidizing acid of an 8% nitricacid solution held at 90° C., and a passive film was formed. The samplewas finally washed in water and dried. TABLE 1 C Si Mn P S Cu Ni Cr MoAl N B 0.081 0.50 0.12 0.014 0.001 0.30 8.0 19.1 0.49 0.08 0.005 0.60

[0062] By way of comparison, a same austenitic stainless steel productin the composition shown in Table 1 was treated by the surface treatmentmethod of etching the surface by an acidic aqueous solution firstinitially researched by the inventors (hereinafter called the priorart). That is, an identical sample as in the embodiment of the inventionwas prepared, and the surface of the sample was etched by immersing thissample for 2 minutes in an acidic aqueous solution of a 10% nitric acidsolution and a 4% hydrofluoric acid held at 60° C. This sample wasfurther immersed for 10 minutes in an oxidizing acid of an 8% nitricacid solution held at 90° C., and a passive film was formed.

[0063] B. Measurement of characteristics

[0064] In these samples, the contact resistance and passive stateholding current density at 0.9 V were measured. Results of measurementare shown in FIG. 5. The contact resistance is a through-resistancemeasured by applying a surface load of 5 kgf/cm² on two overlaid pliesof separators (anode side and cathode side) using a resistance meter.The passive state holding current density refers to the current densitycorresponding to the rate of corrosion when the oxide forming speed ofthe stainless steel of the base material becoming an oxide and the speedof the surface oxide film being melted to become ions are equalized,that is, when the thickness of the oxide film no longer changes, andthis current density was measured by a constant potential polarizationtest. As is clear from FIG. 5, in the sample treated by the surfacetreatment method of the invention, the contact resistance is far smallerthan in the prior art, and the passive state holding current density at0.9 V is larger.

[0065] Using this sample as separator, unit cells of fuel cell wereprepared, and ten unit cells were laminated, and a fuel cell stack wasfabricated. The fuel cell stack was installed in a vibration machine,and changes in contact resistance were observed in a same vibration mode(1.1 G, 30 Hz) as in an actual vehicle. Results are shown in FIG. 6. Asis clear from FIG. 6, in the embodiment of the invention, the contactresistance was low, and there was no change at all up to 3000 hours. Incontrast, in the prior art, the contact resistance was high andincreased with the passing of time. This is because the conductiveinclusions fell off due to vibration.

[0066] Using these fuel cells, power was generated, and the currentdensity at 0.7 V power generation of unit cells was measured from startof power generation until 3000 hours. Results of measurement are shownin FIG. 7. As seen from FIG. 7, only at the start of power generationwas the current density similar in the embodiment of the invention andthe prior art; however, that of the prior art began to decline suddenlyin current density from right after the start of power generation, andfurther declined gradually as time passed. It is also believed to be dueto fall off of the conductive inclusions.

[0067] Using the ferritic stainless steel product shown in Table 2,samples were prepared and tested similarly, and similar results asmentioned above were obtained. TABLE 2 C Si Mn P S Cu Ni Cr Mo Al N B0.081 0.50 0.12 0.014 0.001 0.30 0.54 19.1 0.49 0.08 0.005 0.60

1. A method of surface treatment for stainless steel products for fuelcells, being applied in a manufacturing method of stainless steelproducts for fuel cells comprising, by weight, C: 0.15% or less, Cr: 17to 36%, and B: 0.005 to 3.5%, comprising a first step of forming apassive film preliminarily on the surface of the stainless steel productby an oxidizing acid, a second step of protruding one or more types ofconductive inclusions M₂₃C₆ type carbide, M₂₃(C, B)₆ type boron carbide,and M₂B type boride on the surface by etching the passive film in anacidic aqueous solution, a third step of forming a passive film on thesurface other than the inclusions protruding on the etched surface by anoxidizing acid, and a fourth step of washing in water and drying.
 2. Amethod of surface treatment for stainless steel products for fuel cells,being a method of surface treatment for austenitic stainless steelproducts for fuel cells comprising, by weight, C: 0.15% or less, Si:0.01 to 1.5%, Mn: 0.01 to 2.5%, P: 0.035% or less, S: 0.01% or less, Al:0.001 to 0.2%, N: 0.3% or less, Cu: 0 to 3%, Ni: 7 to 50%, Cr: 17 to30%, Mo: 0 to 7%, B: 0.005 to 3.5%, and balance: Fe and inevitableimpurities, with the contents of Cr, Mo, and B satisfying the followingformula, comprising a first step of forming a passive film preliminarilyon the surface of the austenitic stainless steel product of thespecified composition by an oxidizing acid, a second step of protrudingone or more types of conductive inclusions M₂₃C₆ type carbide, M₂₃(C,B)₆ type boron carbide, and M₂B type boride on the surface by etchingthe passive film in an acidic aqueous solution, a third step of forminga passive film on the surface other than the inclusions protruding onthe etched surface by an oxidizing acid, and a fourth step of washing inwater and drying: Cr(%)+3×Mo(%)−2.5×B(%)≧17 wherein element symbolsdenote contents (wt. %).
 3. A method of surface treatment for stainlesssteel products for fuel cells, being a method of surface treatment forferritic stainless steel products for fuel cells comprising, by weight,C: 0.15% or less, Si: 0.01 to 1.5%, Mn: 0.01 to 1.5%, P: 0.035% or less,S: 0.01% or less, Al: 0.001 to 0.2%, N: 0.35% or less, Cu: 0 to 1%, Ni:0 to 5%, Cr: 17 to 36%, Mo: 0 to 7%, B: 0.005 to 3.5%, and balance: Feand inevitable impurities, with the contents of Cr, Mo, and B satisfyingthe following formula, comprising a first step of forming a passive filmpreliminarily on the surface of the ferritic stainless steel product ofthe specified composition by an oxidizing acid, a second step ofprotruding one or more types of conductive inclusions M₂₃C₆ typecarbide, M₂₃(C, B)₆ type boron carbide, and M₂B type boride on thesurface by etching the passive film in an acidic aqueous solution, athird step of forming a passive film on the surface other than theinclusions protruding on the etched surface by an oxidizing acid, and afourth step of washing in water and drying: Cr(%)+3×Mo(%)−2.5×B(%)≧17wherein element symbols denote contents (wt. %).
 4. A method of surfacetreatment for stainless steel products for fuel cells, being applied ina manufacturing method of bright annealed stainless steel products forfuel cells comprising, by weight, C: 0.15% or less, Cr: 17 to 36%, andB: 0.005 to 3.5%, comprising a first step of forming a passive filmpreliminarily on the surface of the bright annealed stainless steelproduct by an oxidizing acid, a second step of protruding one or moretypes of conductive inclusions M₂₃C₆ type carbide, M₂₃(C, B)₆ type boroncarbide, and M₂B type boride on the surface by etching the passive filmin an acidic aqueous solution, a third step of forming a passive film onthe surface other than the inclusions protruding on the etched surfaceby an oxidizing acid, and a fourth step of washing in water and drying.5. A method of surface treatment for stainless steel products for fuelcells, being a surface treatment method of bright annealed austeniticstainless steel products for fuel cells comprising, by weight, C: 0.15%or less, Si: 0.01 to 1.5%, Mn: 0.01 to 2.5%, P: 0.035% or less, S: 0.01%or less, Al: 0.001 to 0.2%, N: 0.3% or less, Cu: 0 to 3%, Ni: 7 to 50%,Cr: 17 to 30%, Mo: 0 to 7%, B: 0.005 to 3.5%, and balance: Fe andinevitable impurities, with the contents of Cr, Mo and B satisfying thefollowing formula, comprising a first step of forming a passive filmpreliminarily on the surface of the austenitic stainless steel productof the specified composition by an oxidizing acid, a second step ofprotruding one or more types of conductive inclusions M₂₃C₆ typecarbide, M₂₃(C, B)₆ type boron carbide, and M₂B type boride on thesurface by etching the passive film in an acidic aqueous solution, athird step of forming a passive film on the surface other than theinclusions protruding on the etched surface by an oxidizing acid, and afourth step of washing in water and drying: Cr(%)+3×Mo(%)−2.5×B(%)≧17wherein element symbols denote contents (wt. %).
 6. A method of surfacetreatment for stainless steel products for fuel cells, being a surfacetreatment method of bright annealed ferritic stainless steel productsfor fuel cells comprising, by weight, C: 0.15% or less, Si: 0.01 to1.5%, Mn: 0.01 to 1.5%, P: 0.035% or less, S: 0.01% or less, Al: 0.001to 0.2%, N: 0.35% or less, Cu: 0 to 1%, Ni: 0 to 5%, Cr: 17 to 36%, Mo:0 to 7%, B: 0.005 to 3.5%, and balance: Fe and inevitable impurities,with the contents of Cr, Mo, and B satisfying the following formula,comprising a first step of forming a passive film preliminarily on thesurface of the ferritic stainless steel product of the specifiedcomposition by an oxidizing acid, a second step of protruding one ormore types of conductive inclusions M₂₃C₆ type carbide, M₂₃(C, B)₆ typeboron carbide, and M₂B type boride on the surface by etching the passivefilm in an acidic aqueous solution, a third step of forming a passivefilm on the surface other than the inclusions protruding on the etchedsurface by an oxidizing acid, and a fourth step of washing in water anddrying: Cr(%)+3×Mo(%)−2.5×B(%)−17 wherein element symbols denotecontents (wt. %).
 7. The method of surface treatment for stainless steelproducts for fuel cells of any one of claims 1 to 6, wherein theoxidizing acid contains at least 2 to 30% of nitric acid.
 8. The methodof surface treatment for stainless steel products for fuel cells of anyone of claims 1 to 7, wherein the acidic aqueous solution contains atleast 2 to 20% of hydrofluoric acid and 5 to 20% of nitric acid.